In the case of signal transmission through a long-haul optical fiber, there is a case that a signal is degraded due to occurrence of optical noise caused by optical amplification on an optical transmission line, and noise caused in a receiver, and thereby a bit error (in the case of a binary signal, a change of value from 0 to 1, or 1 to 0) occurs. Therefore, an optical transmitter adds an error correction code to a transmission signal and transmits the transmission signal including the error correction code, and an optical receiver detects number of error bits included in a received signal, and corrects the error bit on the basis of the error correction code.
Since the error correction code is usually added every one frame, detection of the number of error bits and correction of the error bit are carried out every one frame. However, in the case of optical transmission, number of the frames transmitted in one second is vast. For example, in the case of OTU (Optical channel Transport Unit) 4, number of the frames, which are transmitted in one second, is 856,388. Therefore, when outputting a number of error bits per frame to an outside, there is a possibility that an amount of output data becomes large and the large amount of output data causes a processing delay, or the like.
In order to make the amount of the output data small, a method of integrating the number of error bits, which are detected every one frame, in a unit of time (for example, one second), and outputting the integration result to the outside is conceivable.
For example, according to a method which is described by a patent literature (hereinafter, abbreviated as PTL) 1, a number of error bits, which occur in a predetermined monitoring period of time, is integrated, and it is determined that degradation is detected when a degradation detection condition is satisfied successively M times on the assumption that the degradation detection condition is satisfied once when an integration value exceeds a threshold value.
Moreover, as a cause of occurrence of the error, influence of steady noise occurring in a transmission system, and influence of instantaneous disturbance or the like are conceivable. In the case of the error due to the influence of the steady noise, a temporal change in the number of error bits is gradual, and the bit error is distributed at random. On the other hand, in the case of the error due to the influence of the instantaneous disturbance or the like, the number of error bits increases only in some frames. As mentioned above, it is possible to estimate the cause of the error by knowing whether the bit error occurs steadily or instantaneously.